In recent years, from the viewpoint of energy saving, light-emitting diodes (LEDs) have been used as light sources for lighting, in place of fluorescent lights, halogen lamps, and the like.
When a surface to be irradiated is irradiated using a light-emitting diode, illuminance varies greatly between a location immediately below a light source (light-emitting diode) and a location separated from the light source. Therefore, when a wide surface to be irradiated is irradiated using one light-emitting diode, illuminance varies greatly between a location immediately below a light source and a rim part of the surface to be irradiated. As a method of uniformly illuminating a wide surface to be irradiated using a light-emitting diode, there has been consideration of the dense arrangement of a plurality of light-emitting diodes. However, such a method is not preferable from the viewpoint of energy saving.
In addition, as another method of uniformly illuminating a wide surface to be irradiated using a light-emitting diode, there has been consideration of the expansion of the distribution of light emitted from the light-emitting diode using a lens (for example, see Patent Literature 1). Patent Literature 1 discloses a light emitting element unit including a light emitting element, and a lens unit that expands the distribution of light from the light emitting element. The lens unit includes an incidence surface on which the light from the light emitting element is incident, and an emission surface from which the light incident thereon from the incidence surface spreads out. The lens unit has a rotationally-symmetrical shape (circularly-symmetrical shape) using an optical axis of the light emitting element as a central axis. Therefore, the lens unit has a circular shape when seen in a plan view. It is possible to uniformly irradiate a wide surface to be irradiated with light from a light emitting element to a certain extent, by using the light emitting element unit disclosed in Patent Literature 1.